摘要:
A technique and system for accurate determination of differential propagation delays in fiber-optic circuits. The method includes providing a sinusoidally modulated optical signal to each of two waveguides defining optical paths. The optical signals received from the optical paths are combined to form a reference output signal which has a null waveform whenever the propagation delay between the optical signals contains an odd number of half periods of the optical signal waveforms. The difference in the sinusoidal modulation frequency producing a first and second null or constant waveform in the reference signal is determined. This difference value between adjacent frequencies forming the null or constant waveforms comprises the inverse of the difference of signal propagation delay in the two optical paths. Accuracy is improved by measuring the sinusoidal modulation frequencies corresponding to first and second waveforms which are not formed by adjacent frequencies. The difference between those nonadjacent frequencies is divided by the difference in the reference waveform orders of the null reference waveforms to obtain the inverse of the differential propagation delay. Further accuracy is achieved by measuring at least one of the waveform nulls at a high waveform order. Still further accuracy is achieved by monitoring the reference waveforms on a network analyzer and using a frequency synthesizer to more precisely match and identify the modulation frequencies corresponding to the null reference signal waveforms. A system is disclosed for implementing this technique optionally using optical sources having a short coherence length. Mathematical relationships are disclosed for use with measured values in obtaining further improved accuracy.
摘要:
A fibre optic lattice filter having a transfer function wherein the poles and zeros are adjustable independently of each other. The filter comprises a cascaded configuration of recursive and non-recursive fiber optic lattice filters. In one preferred embodiment, an all-pass fiber optic filter is formed by cascading all-pole and all-zero lattice filters, and by processing the resulting filtered output signal in a subtractive detection system. This detection system produces a signal which represents the difference between two signal outputs provided by the all-zero filter section, and which is adjustable in magnitude, thereby providing an overall filtering function which is capable of handling both positive and negative valued input signals.
摘要:
A fiber optic recirculating memory comprises a splice-free length of optical fiber which forms a loop that is optically closed by means of a fiber optic coupler. The coupler couples an optical signal input pulse to the loop for circulation therein, and outputs a portion of the signal pulse on each circulation to provide a series of output pulses. A pump source is included to pump the fiber loop with a pump signal having sufficient intensity to cause stimulated Raman scattering in the fiber loop, and thereby cause amplification of the circulating signal pulse. The fiber characteristics, coupler characteristics, and a pump power are selected to yield a Raman gain which compensates for the total round-trip losses in the fiber loop, so as to provide an output pulse train of constant amplitude pulses. The invention may be implemented utilizing a standard coupler with a pump signal modulation technique. The pump signal is input to the loop as a series of pulses having a duration and a periodicity chosen to prevent overlap of recirculating pump pulses with input pump pulses until at least two circulations of the loop, which minimizes pump power fluctuations and will therefore enhance output signal stability.
摘要:
A single mode optical fiber switch having a base (70) and a laterally slidable top (91). The base is composed of a quartz block (70) with a slight radius of curvature along its length supporting a silicon substrate (66) having a plurality of parallel v-grooves (68). The grooves are fitted with single mode optical fiber segments (64) and lapped to create a flat coupling surface (82) extending laterally across the base. The top (91) is composed of a quartz block with a single v-groove (93) holding a segment of single-mode fiber (95). This fiber (95) is lapped to create a flat coupling surface matable with the coupling surface on the base. The base (70) and top (91) are placed together such that the top fiber (95) may be selectively slid into and out of coupling alignment with any of the fibers (64) in the base (70) to form a discretely variable delay line. A single length of fiber (121) is wrapped around the base ( 70) in helical fashion with each loop being secured in one of the v-grooves (68). By coupling the top fiber (95) with different loops of the base fibers (121), different amounts of delay can be obtained. The delay line can be used to provide variable frequency response for notch and transversal filters.
摘要:
There is disclosed a high speed pulse train generator for generating a train of pulses having arbitrarily close spacing. The apparatus consists of a generator loop comprised of a fiber optic waveguide looped through a directional coupler with the output fiber serving as the input fiber for a similarly structured multiplexer loop. The lengths of the two loops are adjusted such that the time difference in the propagation times of light around the respective loops is small compared to the time of propagation around either loop. The times are adjusted to obtain any arbitrary spacing of the pulses in the output pulse train which is comprised of interleaved pulse trains resulting from each pulse input to the multiplexer loop. Also disclosed is a single loop embodiment for bidirectional data rate transformation and methods of using all the embodiments.
摘要:
Apparatus and method of manufacture for coupling optical power between two strands of fiber optic material in a given direction of propagation. The coupler employs generally parallel, intersecting strands of fiber optic material having the cladding removed on one side thereof to within a few microns of the fiber cores in the region of intersection to permit light transfer between the strands.
摘要:
A passive, frequency selective, fiber optic multiplexer, comprises a directional coupler in which a pair of single mode optical fibers are accurately positioned to provide evanescent field coupling, typically by polishing a portion of the cladding from each of said fibers to place the respective cores of said fibers within the evanescent field of light in the other fiber. The coupling efficiency of a coupler constructed in this manner is wavelength dependent, and provides over-coupling, that is, the capability of transferring light, virtually entirely, back and forth between the fibers within the coupler. The wavelength dependent nature of the evanescent field coupling permits multiplexing, specifically between a pair of wavelengths, one of which is coupled in its entirety from a first fiber to a second fiber, and the other of which is essentially uncoupled. By increasing the number of total transfers of the light signals between the pair of fibers, the frequency resolution of the multiplexer may be optimized for light signals of virtually any frequency separation.
摘要:
A pair of small diameter optical fibers are arranged in a side-by-side configuration, the first fiber providing a passive guide for pump light, and the second fiber formed of a material, such as Nd:YAG which has amplifying properties at the signal frequency. The signal to be amplified propagates through the second fiber from the active material, resulting in amplification of the signal. The first fiber is pumped from one or both ends, utilizing a cone shaped rod which is optically coupled thereto. A large diameter beam of collimated pump light enters the cone shaped rod, where total internal reflections on the cone walls compress the beam to a small, fiber-size diameter for absorption by the active fiber. The light input to the cone-shaped rod is produced by plural, high power laser diodes, and is collimated by microlenses. By utilizing the cone-shaped rod to couple light from the high power diode lasers to the pump fiber, high pump power densities can be achieved, yielding increased amplification.
摘要:
A fiber optic sensor comprises a length of optical fiber, forming a loop, and a fiber optic directional coupler for optically closing the loop. The loop and coupler form a resonant cavity for light circulating therethrough. A PZT cylinder, about which the fiber loop is wrapped, is utilized to control the total round trip phase delay of the circulating light, and thus, control the intensity of the optical output signal. The phase delay is adjusted to a point where the optical output signal is at maximum sensitivity to changes in phase. When the fiber loop is exposed to, e.g., acoustic waves, the loop length changes correspondingly, thereby causing the phase delay, and thus, the optical output signal to vary. By detecting variations in output signal intensity, the frequency and intensity of the acoustic waves may be determined. The sensor also includes a feedback system for stabilizing the fiber loop against low frequency thermal drift.
摘要:
A fiber optic rotation sensing interferometer based on the Sagnac effect in which mechanical rotation introduces measurable shifts in the phase of optical signals transversing a closed path. The interferometer includes the closed optical path made up of a multi-turn fiber optic loop, a directional coupler or couplers, and an amplifier. A pulse of electromagnetic radiation is fed into the optical path by means of a pulsed electromagnetic source such as a laser and a beam splitter. The beam splitter splits the pulse into two pulses which undergo multiple circulations about the optical path in opposite directions. The directional couplers non-destructively sample the two pulses after each pass therethrough and send the pair of pulse samples to the beam splitter once each circulation. The beam splitter sends these pulse pair samples to a detector and signal processor once each circulation. The detector measures the instantaneous relative phase shift between the two pulse samples in each of the multiple pulse pairs and outputs phase information once each circulation. This phase information is converted by the signal processor into the angle of rotation or into rotation rate of the interferometer.